Direct reading vernier heterodyne very high-frequency meter



1954 L. s. CUTLER ET AL DIRECT READING VERNIER HETERODYNE VERYHIGH-FREQUENCY METER 2 Sheets-Sheet 2 Filed June 5 1951 ENE Qm\ Nv gimb-I 0llll ZME m n w 4 M0 d mfl n f 2 w L m ATTORNEY Patented Oct. 12,1954 UNITED STATES PATENT OFFICE DIRECTv READING VERNIER HEIPERO'DYNEVERY HIGH-FREQUENCY METER- of. California;

Application June 5, 1951', S'erialN'o. 230,062

11 Claims. 1'

This invention relates generally to improvements in frequency measuringand signal generating devices.

More specifically, this invention relates to an improved wide rangefrequency meter and signal generator of the heterodynety-pe adapted toserve the dual function of accurately measuring and directly indicatingthe frequency of a signal of unknown frequency, and" of generating adesiredfrequency to a high degree ofaccuracy;

A- primary object of the invention is to provide a direct readinghet'erodyne type frequency meter and signal generator wherein thefrequency of an unknown VHF signal, or of a generated VHFsignal, maybedirectly read to an accuracy of at least .005%' withoutemployementofel'aborate or cumbersome calibration charts, reductiongearing or band-spread devices. This facilitates the design withparticular reference torelativel'y small and portable instruments, butthe same features are of course applicable to larger and more accurateequipments of the same type.

Another object is to provide an improved method of devising a matchingfrequency from twocomponent frequencies; one of'whi'ch is stabilized toa high degree of accuracy and is employed both to produce a relativelyhigh frequency carrier and to calibrate a tunable low frequency signalwhich modulates the stabilized high frequency in such a manner as toproduce a high difference frequency, which in turn may be used to' matchanunknown frequency orto act'as a signal to the frequency of which maybe. accurately determined by direct reading of the component frequenciesemployed to initiate said difference frequency;

It has heretofore been. proposed to incorporate, in: a frequency meter,signal generator, or similar equipment, a highly stabilized referenceoscillator, such as a crystal-controlled: oscillator, and to utilize theaccurately determined harmonies of the fundamental frequency ofsuchoscillator to establish a plurality of frequencies which can be comparedwith an unknown signal in. order to measure the frequency of' thelatter. Also, in order to permit the measurement of frequencies lyingbetween the. harmonics of the crystal oscillator, it has been proposedto provide a. so-called interpolation oscillator whose output frequencyrange is sufiicient to cover the band between adjacent crystal harmonicfrequencies,

and to mix the output of this variable-frequency oscillator with theappropriate crystal harmonic in order to produce side-band frequenciescapable-of' matching any unknown: frequency within the range of theinstrument. Such prior proposals, however; have been subject to variousdisadvantages, among which are the difiiculty of filtering out theunwanted modulation products between the crystal harmonic and the outputof the variable interploation oscillator, the necessity of referring totables and charts for translation ofthesettings of the oscillators intoactual frequency values; the ambiguity which exists because of thedifficulty of determining which crystal harmonic is being utilizedateach time, 'as well as which of the modulation products is beingcompared" with the unknown frequency; and the; relativelycomplex'equipment and circuitry required by such prior designs;

The present invention incorporates ways and means by which a singlestabilized and accurate reference oscillator serves as the source of aplurality of accurate harmonic frequencies which are utilized as carrierfrequencies capable of being modulated by the signal'from avariable-frequency oscillator to provide asignal of any desiredfrequency, and in which the modulation frequency itself is alsoaccuratelydetermined by comparison with the reference oscillator. Inorder toeliminate the effect of undesired harmonics, the unknown signalis first compared with the variable'frequency generated by a transferoscillator whose tuning" control is linked to the tuning control of theharmonic amplifier or generator excited by the reference oscillator. Theact of adjusting the transfer oscillator to-match the unknown signalthus aceomplishes two-things simultaneously: it gives a direct readingof" the order orapproximate magnitude of the unknownsignal; for example,its dial may indicatedirectly the first tWosignificant figures of thedesired measurement, and it also automatically adjusts the tuningcontrol of the harmonic generator excited from the reference oscillatorso that the harmonic appropriate to the order of magnitude beingmeasured appears in the output of the harmonic generator with muchhigher amplitude than do any of the otherharmonics not needed" for theparticular measurement under consideration. This completely eliminatesany ambiguity as to whether the proper harmonic is being utilized.

Inasmuch as the modulation products between the selected harmonic of"the reference oscillator and the output of the variable frequencyinterpolation oscillator would normally include, in addition tothese twofrequencies themselves, the sum and differencefrequencies thereof, aswell asfrequencies having values twice those of the said twofrequencies, the result of merely mixing the two frequencies would be toprovide an output including several frequencies any one of which mightbe that desired and necessary to Zero-beat the unknown frequency or thefrequency of the transfer oscillator as matched therewith. The problemof filtering out these unwanted modulation products is a serious one,involving numerous expensive filters which increase the cost of theequipment as well as representing space and weight which areundesirable, especially in an instrument intended for portable use. Thepresent invention eliminates the need for these filters by an ingeniouscombination of the use of a lattice-type modulator for combining theselected harmonic frequency and the output of the interploationoscillator which inherently suppresses both of those frequencies assuch, as well as the double frequencies, and

the selection of the tracking relationship between the transferoscillator and the harmonic generator, and the frequency range of theinterpolation oscillator, so as automatically to eliminate either one ofthe sum and difference frequencies. Since the frequency range of theinterpolation oscillator need encompass only the spectrum length betweenadjacent harmonics of the reference oscillator, its dial or dials can becalibrated directly with sufficient accuracy to indicate the remainingsignificant figures of the desired or measured frequency withoutmechanical complications and without the need for charts, tables or thelike.

The invention further includes a particular arrangement of the dialscontrolling the harmonic generator and the interpolation oscillator sothat their respective readings appear side by side in such a way as topermit the entire frequency reading to be obtained at a glance andwithout calculation of any kind.

A further object of the invention is to provide a frequency meter orsignal generator of the above general type, in which certain of thecomponents have multiple functions, whereby the size, weight andcomplexity of the device are reduced, together with a switching systemby which the various circuits can be adjusted readily for theperformance of the necessary functions of generation, measurement andcalibration, by the operation of a single control.

The above and other objects and advantages of the invention, and apreferred way of accomplishing the same, will best be understood fromthe following detailed specification of a selected and exemplaryembodiment thereof, reference being had to the accompanying drawings.However, it is to be understood that certain of the advantages of theentire invention can be obtained by applying the same principles toother specific combinations of components; for example, the novelprinciples of the invention are capable of producing an accurately knownfrequency for any purpose whatever, and not only for measurementpurposes. Thus, the principles hereof may 'be adapted to the control ofthe exciter oscillator of a radio transmitter or the like, providing avariable frequency transmitter having very simple controls, and forother equivalent purposes. Moreover, while the invention as exemplifiedherein utilizes the harmonics of a single crystal-controlled oscillator,it is equally possible to utilize a plurality of accurately knownfundamental frequencies, crystal-controlled or otherwise. Such changesin design and application do not necessarily depart from the principlesof the invention, and are to be understood as included in the scopethereof except as it may be limited by the claims appended hereto andforming a part of this application.

In the drawings,

Fig. 1 is a block diagram of the invention as embodied in a frequencymeasuring equipment, and indicating also the ganged relationship of thetuning controls of the transfer oscillator and the harmonic generator,and

Fig. 2 is a complete schematic diagram of the preferred embodiment ofthe invention.

Referring to Fig. 1, crystal oscillator It) serves as a frequencystandard, and hence is designed for a high degree of temperaturestability, and conveniently, but not necessarily, is arranged to operateat a frequency of one megacycle and to be readily adjustable to exactlythat frequency by reference to any primary or secondary standardavailable. The output of oscillator H] is fed to a tuned crystalharmonic multiplier I2, the tuning condenser in the tank circuit ofwhich is gang-connected to the tank tuning condenser of a high frequencyelectron-coupled transfer oscillator [4 for synchronized trackingtherebetween for a purpose later described. A selected harmonic outputsignal from multiplier I2 is fed to a balanced modulator is which alsoreceives the output signal from an auxiliary or interpolation oscillator18 and operates in such a manner as to suppress the carrier frequencyfrom multiplier l2 and the modulating signal from oscillator l8, and topass on only the sum and difference signals resulting from the mixing ofthe relatively high frequency signal from multiplier l2 with therelatively lower frequency signal from oscillator l8 for matching withthe output signal of transfer oscillator M, as evidenced by a zero-beatcondition established in detector 20.

Detector 20 is employed initially (when the device is used for measuringan unknown frequency) to develop a difference frequency voltage betweenan unknown input signal (received via a conduit 22 or antenna 24) and amatching frequency produced by transfer oscillator I4, after which theinput signal is disconnected, and the difference frequency frommodulator I8 is zero-beat with the output of transfer oscillator 14. Thesignal from detector 2i] is fed through a conventional audio amplifier2B, the output of which may be connected to headphones or other meansfor indication of the zero beat condition.

The circuit details, as schematically shown in Fig. 2, cover thecomponents shown in the block diagram of Fig. 1, the component circuitsbeing bracketed with like numerals to facilitate reference. Transferoscillator I6 is of the conventional electron-coupled type wherein thescreen grid 43 of pentode tube H4 serves as the plate of a Hartleyoscillator, with tube H4 serving as both an oscillator and an amplifier.The fundamental range of this oscillator may be, for example, from 20 to40 megacycles per second (mc.), with harmonics through the twelfth beingemployed for complete coverage from 20 to 480 me. The plate load circuitis left untuned for increased harmonic output. The amplified outputsignal is fed through blocking capacitor 58 to detector circuit 23comprising resister 52, capacitor 53, and a germanium crystal diode 54,the circuit being grounded at 55. The manner in which signals areintroduced to or fed out of the system, through coaxial line 22.

moor-me will later he described im cozmc'ctiimi with: theswitchingamgementi Th harmonic'v multiplier: 1 2f include-s apent'odetube: {Mand is of conventional design withthe exception that its tunedplate circuit includes I a: variable condenser mechanicallyconnected, asshown at 61 for: gangtuning with the variable condenseroftransfer-oscillator M wondensers- 61F and B5 areso designe'di: withrespect ta oscillator Ill includes a pentode tuhe WU; connected as atri'ode; and emplbysa-frequency standard crystal' 65', subject toonlyslight frequency shift with" temperature variations,- the" circuit beingprovided with a trimmercondenjser' 6%? for "correction of the crystalfrequency tothat of a frequency standard; The plate Tll of'tubel-Oll isconnected, through blocking" condenser-T123130 the controhgridlfloftube-lH. in a known manner;

Auxiliary or interpolation oscillator l8, employihg a triode H8; mayalso beof the, wellknown Hartley type; with a' cathode 16" above groundpotential; and is de'signedi in the embodiment' being described,'togenerate any dosiredfrequency in therange from" 1 to 2 megacycles'per second. Plate Tris-energized through conductor 18 which also leadsthrough resistance 79- and high frequency choke" 80 toithe-plate oranode oftube H2. Cathode'lfit of auxiliaryioscill'ator tube '8 isconnected: through resistor 84 to a center tap 8'6= of the secondarywinding" 8! of the input transformer 88 ofthe ha'lancedlattice modulatoror mixer H3. Modulator It is of the lattice type employing pluralgermanium crystal diodes 90, 9t, 92- and. 93- respectivelyyin the mannershown; for operation'in aknown manner. Amodul'ator' of this type isshown and well describedinFrench Patent No; 739303, granted October 24,1932. V The primary" winding 95'' of output transformenafifli'sgrounded'at its midpoint to provide a return circui'tt'o auxiliaryoscillator f8: Secondary or output'wi'ndirig: 9 8

of transformer 96 is connected by lead 99; to a junction l t 5between'the' output circuit oftransfer o'scill'atorl l, the coaxiallea'd -in'2 2i, and the detector circuit 2U; Thus, by means of switchesto be described, thedesired combinations'of signals may be presented to"detectorxill toenable comparison thereof method.

As abovementioned; an unknown'signal'canbe bythe usual zero-beatintroduced to the system, or asignalof.. known frequency obtainedtherefrom, by means of a and: the auxiliary oscillator-t8 are:deenengized (by interruptionof their platerand screen voltages).,. andthe conductor. 2 2i and tnamiferoscillator l4 are; connected;totthe-input" of-...detector [interpolation oscillator:

20'; 130? enable: comparison: of: the-output oi said oscillator .Hbwith.an unknowrcsignall; arriving through. thatconductom. or: (whenrusing:thedervice aslaisignak generatorl, to enable the? output of. oscillator:lit to be supplied. to: the conductor for exsterrxetlziise. Audioamplifieraztsiseenergized inthisuswitch; position sin as-to enahlie:detection of the heat: note, ifzazny, the outputzof detector: 202

the gang: switch set Read: position,

conductor: 2-2 .isladisconneeted: from the. signal input (detector inthe. crystatoscillator: I10 muttiplier k2,. transfer oscillator 14: and:auxiliary oscillator 158 arerenergized,.and the: signal outputsofcthetransfer. oscillator: l4 modulator: t6 are applied to deteetonZOgJto enable. the transfer oscillatontmberset at-theidentical-lfrequency gen erated by the; modulation; ofthe: harmonic the;multiplier by the output/of the auxiliary or Ire the-Calibrateposhtiiorn of. the svoitcl'r the fundamental frequency produced. byreference oscillator" lull:- is applied to the'deteotor, alongWithulihfi output. of the autooscillator to. enable: calibratiom' of thedial iflf the latter; at e checic points established by the crystaiborireference: oscillator; While the harmonic: multiplienstag e still;energized? at this time, its output does notconiuse the: operatorbecause at: any positions ofi dial. bill the. harmonic output from: II2: is ofrwe'ry much; smaller magnitime than that. ct: oscillator Inthe" Modulate positiong. which: is: employed when. it; is desired tfllgenerate: an. accurate" frequency for external use: and carrying:anz-audim-fnequenny modulation component; the: CGIIIIECblDI-XSZ'IIB' thesame asuim thezBeattpositrom. except that'audio: amplifiert Z6 is:res-connected; as amaudio oscillator with: its. output employerli to:modulate theplate and screen: of. the" transfer oscillator M...

The manner im which the? above: functions are obtained w-illlbeiWGlIllIIIfiEISStOOdl by-"examinmgin Fig;.:2;.thesixswitches ism; I32;IHLJMi-Htzand It; Each: comprises a rotatable; blade on the like which:cam contact any one": of flour terminals, these being; desfgnatedby theletters wto d used. as subscripts: the corresponding reference: numeralsforth-e switches in theafollowing description. Switch I30 operates to:direct the outputs ofrcrystal oscillator i302 and. auxiliaryoscillators.lo tm detector tattzjimction. I205) when in. position: c .(lcalibrateli, and to interrupt this connectiomin all; other ptositionsa.Terminalsa,

b and. d; aregroundezii im these: other positions,

- but the: outputs" of 1 the: two oscillators referredito are notshuntedi tosgroundbecause the lead. from SWitChL 341 to each: attheoscillator' output circuits includes: an' appropriates. capacitor;Switch lt3 2 connectsthe sig-nallinput and: output circuit; toconductor". 22,-.and; heneeztoithe transfer: oscillator M1 and.detector- 2 0 in: positions; or and d, for: Beat and. l\ Iodulate,. butdisconnects. circuit; in: the intervening Readand Qlal-ibratapositionsaSwitch llt lrconnects the-audio amplifier: 26 man output ofidetector. 205in; positionsia', a ancl'ac,.fon'beat:-note detection-,; but inposition: (1 (Moth-late), the switch connects thelower endzot:the-output transformer'of the. amplifier to itscontrol grid for.oscillator? operation. The. output. circuit; of

thistransformer is conn'ectedvby switch; lht to the headphone jack 28ins'positions 12;, b ando, but is disconnected in: the Modulate:position...

Switch; r385 hasitsmovable blade or: arnnconnected to the: plate" andscreen, electrodes ofithe transferoscillator: Ma and in positions: a.-and: b completes theplatasupply voltageto: thisos- 'exclusion of othercrystal harmonics.

cillator from the batteries or power supply plug. In position 0, thisswitch disables transfer oscillator by interrupting its plate supp y(for C'alibrate operation), and in position at (Modulate) the switchsupplies to the transfer oscillator a plate and screen voltage derivedfrom the amplifier/oscillator 26, now connected as an oscillator andaudio modulating the output of the transfer oscillator. Finally, switchI40 operates to interrupt the supply of plate voltage to crystaloscillator I8, multiplier I2 and auxiliary oscillator I8 in positions aand (2 (Beat and Modulate), a bleeder resistor being connected to theplate supply line in these positions to maintain the voltage supplied tothe other component circuits at approximately the same level. A Twopole,two-position switch I42 is provided for selecting filament, plate, andscreen potentials either from internal batteries I46 and I48, or

from an external source led in through connector I50. The heaterelements of the various tubes are illustrated at I52, with the plate andscreen circuits being connected either directly or through droppingresistances to the plate voltage supply as shown. -An on-off-standbyswitch I53 enables the heaters to be kept energized, with the plate andscreen voltages disconnected, in a well-known manner.

The tuning of oscillator I4 results in the simultaneous selection of theresonance frequency of the multiplier circuit I2 and hence theparticular harmonic of crystal controlled oscillator III to be employed,and thus the carrier" frequency provided by harmonic generator I? to bemodulated by the signal of auxiliary oscillator I8 in the modulator isautomatically set, to the practical The fixed and ganged adjustablecomponents of the tuning circuits of oscillator I4 and multiplier i2 aredesigned to effect tracking of the tuned circuit of harmonic generatoror multiplier I2 at 1.5 megacycles above the adjusted frequency ofoscillator I4, so that the carrier fed from harmonic generator I2 tomodulator It will lie between 1 and 2 mo. above the adjusted frequencyof oscillator I4, and hence the same amount above the frequency of theunknown signal when zero beat is achieved.

The function of auxiliary oscillator I8 is to produce an output signal,variable through a fre quency range of one to two mc., to be heterodynedwith the frequency of the harmonic generator I2 and thus to supply adifference frequency which will match the frequency of the transferoscillator and of the input signal at a particular dial settingindicated on a geared-down dial to permit accurate reading directly indecimal fractions of megacycles. This is accomplished by utilizing theoutput of the auxiliary oscillator to modulate the output of theharmonic generator, used as a carrier source, the modulating signal andcarrier being fed to the balanced lattice modulator or mixer l6 whereinthe frequency of the carrier is caused to beat with the frequency of theauxiliary oscillator in such a manner that the difference frequencyoutput will equal exactly the frequency of transfer oscillator I4 whichhas been matched previously with the frequency of the unknown signal. Itis apparent that since the frequency of the harmonic multiplier I2 isabove the frequency of oscillator I4, a zero-beat condition can beeffected between the signals from oscillator I4 and modulator I6 only bythe difference signal between multiplier I2 and low frequency oscillatorl8, and that the sum signal may be ignored. The circuit arrangement foraccomplishing this result is explained below in greater detail.

Operation of the device as a heterodyne frequency meter, is as follows:With gang switch I30 to I40, herein termed the function switch, set atthe Beat position, i. e. with its movable arms engaging contacts a,coaxial line 22 is connected, through a second coaxial line and throughpart of a grounded potentiometer I54, line I56, and terminal I58, to asource of unknown frequency, such as from a radio receiver. The unknownsignal is thus fed to the detector circuit 20, and from an examinationof the circuitry and what has already been said it will be seen that thefunction switch has operatively connected the transfer oscillator,detector and audio amplifier circuits while disconnecting plate or anodesupply leads of the crystal oscillator, frequency multiplier andauxiliary oscillator circuits. The transfer oscillator is then tuned,with headphones inserted in jack 28, for a zero-beat condition betweenthe unknown signal and the output of that oscillator, the method ofobserving that condition through audible observation of the detectoroutput signal being well known in the art.

The function switch is now turned to the Read position, i. e. with itsarms engaging contacts b. In this position the unknown signal isdisconnected, but the transfer oscillator I4 remains operative to feed asignal of identical frequency to detector 20. Plate and screen voltagesare now connected to crystal oscillator Ill, frequency multiplier I2 andauxiliary oscillator It, to place those circuits in an operativecondition. As heretofore stated, the tuning of transfer oscillator I4 tomatch the frequency of the unknown signal has resulted, by reason of thegang connection between the tunable components of that circuit and thetunable components of frequency multiplier circuit I2, in the selectionof that harmonic of crystal oscillator II) which is between 1 and 2megacycles higher than the frequency of transfer oscillator I4. Sincecrystal oscillator I0 is functioning, the said harmonic thereof whichfalls within the resonance band or selectivity curve of multipliercircuit I2 is amplified in that circuit, with attenuation of all othercrystal harmonics, and is supplied as a carrier, by conduit I64, toprimary winding I68 of the input transformer 88 of balanced latticemodulator I6. Since this carrier is a harmonic multiple of the crystalcontrolled circuit I0, the frequency of which has been adjusted bytrimmer condenser 61 to exactly 1 megacycle and calibrated as laterdescribed, the dial IIll of Fig. 1, which is mechanically connected tothe gangtuned components of circuits I2 and I4, is accurately readable,in whole megacycles, to indicate said carrier frequency. It will benoted that the output signal from transfer oscillator I4 passes throughcapacitor 50 to detector '20, to which is also connected the secondary98 of the output transformer 96 of modulator circuit I6, and since thetwo signals applied to the modulator circuit I6 are not of the samefrequency, the usual sum and difference signals are fed to detector 20.As has been stated, the lattice type of modulator inherently suppressesboth of the original frequencies themselves, and their doubles. Thefunction of auxiliary oscillator I8, as previously mentioned, is toproduce an output signal which can be made to equal the differencebetween the output frequencies of cirshown in Fig. 1.

'rouits and 14 and which :is readable to :a high products being fed todetector 20.

Since the harmonics of a one-megacycle reference oscillator are onemegacycle apart, it would appear that an interpolation or auxiliaryoscillator such as oscillator l8 could cover the range between adjacentharmonics of the crystal frequency if it were tunable from to 1megacycle. As a practical matter, however, this would require anauxiliary oscillator of extremely difficult construction, since itstuning range includes very low frequencies and the achievement of thenecessary degree of stability would require an expensive and unwieldystructure. For this reason, the present invention utilizes an auxiliaryoscillator, tunable through a higher "range, -and for the embodimentbeing described, the range from 1 to 2 megacyclesis"employed, and thisis the 'reasonfor arranging matters so that the harmonic multiplier 12tracks 1:5 megacyc'les above the transfer oscillator frequency. Theinstrument can still be read directly, because the geared 'd'ial of theauxiliary oscillator reads from .000 to 1:000as the oscillator itself istuned from 2 to '1' megacycles per second. Since it is "the differenicefrequency (lower side-band) output of modulator it which is utilized,the mega'cycle indication of dial 110 will be correct because theharmonic multiplier output is actually from 1 to'2 megacycles higherthan the frequency of the transfer oscillator, the tank circuit of themultiplier being-tuned to a frequency 1:5 mega- 'be obtained-with diallI-Oread-ing some valve between24 and '2-5'megacycle's, as shown inFig. 1. Since multiplier l2 tracks-1.5 meg-acycles above the transferoscillator, the maximum of its selectivity curve or resonance willcenter at about 26-megacycles, and-the 26th harmonic of crystaloscillator I0 will appear strongly in the output;

The sum frequency, which would be 26.000+ (2 or 27.443; cannot beobtained with any setting of dials I12, I14, because the gang'in'g ofthe controls of the transfer oscillator and of the multiplier ensuresthat the 27th harmonic ofthereference crystal oscillator (and all other1-0 harmonics) hasbeen strongly attenuated in the resonant circuitof themultiplier.

- Jt is apparent trot-n the above that the device may be used as asignal generator with the :func- -tion switch set at the Read position.The transfer oscillator is then set at approximately the desiredlfrequency, or as nearly as can be ob- I taine'drfrcm the dial 11:0associated therewith. The auxiliary oscillator is then tuned toindicate,

as read in cormection with the first mentioned dial, the exactfrequency. desired, after which the transfer :oscillator is retuned asnecessary for the zero-beat condition indicative of the particularfrequency selected- Since it is desired to use the crystal standardoscillator to calibrate. the auxiliary oscillator, the third or.Calibrate .position, i. c. with the switch arms engaging contacts c, isprovided for that purpose. this switch lposition input lead -22 isdisconneotedand the outputof crystal oscillaltor It is fed to detector-20 via switch i300, theplate circuit of transfer oscillator It beingopen. at I300 to deenergize that oscillator. ,The fundamental frequency,of oscillator 10 can now be com-pared with the output of auxiliaryoscillator l8. and the dial associated with the latter circuit may becalibrated at 1.000 reading, which actually is1 -mc., the fundamental ofthe oscillator .-l=0. Galibrati-on at .500 and 0.000 readings can alsobe carried out in the same manner, but using second and third harmonics.An adjustable trimmer for adjusting oscillatorlB to zerobeat at thesepoints is shown at, H8. The auxilia-ry oscillator may be suppliedwith acurve for correction of its dial reading, to allow for productiontolerances in the variable capacitor and consequent calibratingcorrections between these check points. Calibration as above describedis of course subject to correction factors from the curve, and shouldre-cal-ibration of the auxiliary oscillator be needed, a 1 to 2 me.frequency source with suitable accuracy should be used.

The above description covers the provision of only three check pointsfor calibrating the low frequency auxiliary oscillator. These particularthree check ,points or marker frequencies are emphasized at the expenseof other potential marker frequencies derivable from the one megacyclereference oscillator l0. This is accomplished because of the fact thatthe signal from the low'fr'equency auxiliary oscillator 1 B is derivedfrom its high Q tuned circuit between the control grid and cathode oftube 1 l8, rather than froin the untuned' plate circuit. 'The inventionis not, however, tobe considered limited to this connection, because ifadditional marker frequencies for calibration of the dial of oscillator[8- are desired, this can readily be accomplished by deriving the outputof oscillator 18 through a suitable network in its platecirc'uit, whichwould vastly increasethe harmonics available for providing'additionalcheck points against the'ref'erence oscillator I0. 'In this case, someadditional care would be needed in operation to insure that the propermarker. frequency corresponding to a givendial position is used. Anotherway in which a larger number of check points could be obtained would beto couple from the auxiliary oscillator l 8-as at present, but replacingthe one megacycle oscillator Ill withthe combination of, say, a 50kilocycle crystal oscillator followed by an additionalharmonicmultiplier to produce a stabilized frequency of 1 me'gac'ycle whichwould be fed to the present harmonic multiplier 12.

This combination would provide marker frequencies for calibration forthe auxiliary oscillator spaced 50 kilocycles apart, and would greatlyincrease the accuracy of calibration of the auxiliary oscillator andhence of the instrument.

The fourth or Modulate position of switch l30l46 is employed forconversion of the device to operate as a signal generator with an audiomodulated output. With contact arm I34 engaging fixed contact d, audiooscillator circuit 28 is converted, through conduit I80 and capacitorI82, into a Hartley audio oscillator, connected through switch terminal13801 and conduit I88 to Heising modulate the output of transferoscillator circuit M.

For frequency measurement of a strong local signal, a pick-up antennamay be plugged directly into the output jack. The function switch isplaced in the Beat position. The knob controlling dial I is turnedslowly in the vicinity of the expected frequency until a beatnote isheard in the phones. For frequencies above the fundamental range of thetransfer oscillator dial I70, a proper multiple is selected.

The signal is carefully zero-beat, after which the function switch ismoved to the Read position. The knob controlling dials I72 and I74 isthen turned to the vicinity of the desired signal. For example, if theleft hand dial, [10, reads midway between 24 and 25, as shown, thesignal will be I.

found in the vicinity of .500 on dials I72 and I'M. When the beat noteis heard, dials I72, I'M are adjusted to zero-beat in the center of themost intense beat note. The correction factor is applied to the readingof dials I72 and :4.

iii, and the reading of these dials, taken in connection with thereading of dial 110, and the whole multiplied by the proper rangefactor, gives the measured frequency to the desired accuracy.

The instrument as described herein utilizes a reference oscillator illhaving a single fundamental frequency, together with its harmonics; itis of course feasible to utilize a plurality of stabilized referenceoscillators of diiferent frequencies, or a single oscillator with aplurality of interchangeable frequency-controlling elements (crystals orthe like) to supply the desired carriers to the modulator IS. The meansfor selecting the desired fundamental would still be gang-connected withthe transfer oscillator M to accomplish automatic correlation and thusto permit the direct reading dial system as shown herein.

The present invention has also been described in connection with atransfer oscillator l4 having a fundamental range running from tomegacycles, which permits covering the entire range from 20 to 480megacycles by use of the second, fourth, eighth and twelfth harmonics,and by applying the appropriate multiplier to the reading of theinstrument. It would be equally possible to use only the fundamentalfrequency of a suitable transfer oscillator capable of covering theentire range; for example, a high frequency oscillator utilizing tuningelements of the butterfly type, or equivalents.

The above and other changes and modifications which fall within theskill of those trained in this art are not to be taken as outside thespirit of the invention, except as the same may be limited by the scopeof the appended claims.

We claim:

1. In a high frequency measuring apparatus, a precision oscillatoroperating at a fixed frequency substantially lower than that of a signalto be measured, a variable oscillator tunable over a range encompassingthe frequency of the signal to be measured, harmonic generator meansconnected to the output of said precision oscillator for generatingoscillations harmonically related to its fixed frequency, a tunableresonant circuit coupled to the output of said generator means, meansfor simultaneously adjusting the frequency of said variable oscillatorand the tuning of said circuit such that the latter resonates at afrequency a predetermined amount higher than the adjusted frequency ofsaid variable oscillator, an auxiliary oscillator tunable over afrequency band at least as great as the frequency of said precisionoscillator and which band includes a frequency equal to saidpredetermined amount, modulator means connected to modulate the outputfrequency of said resonant circuit with the output frequency of saidauxiliary oscillator and to suppress both of said frequencies, and meansfor comparing the lower sideband output frequency of said modulator withthe output frequency of said variable oscillator.

2. In a high frequency measuring and signal generating apparatus, aprecision oscillator operating at a fixed frequency substantially lowerthan that of a signal to be measured or generated, a variable oscillatortunable over a range encompassing the frequency of the signal to bemeasured or generated, harmonic generator means connected to the outputof said precision oscillator for generating oscillations harmonicallyrelated to its fixed frequency, a tunable resonant circuit coupled tothe output of said harmonic generator means, means for simultaneouslyadjusting the frequency of said variable oscillator and the tuning ofsaid circuit such that the latter resonates at a frequency apredetermined amount higher than the adjusted frequency of said variableoscillator, an auxiliary oscillator tunable over a frequency band atleast as great as the frequency of said precision oscillator and whichband includes a frequency equal to said predetermined amount, modulatormeans connected to modulate the output frequency of said resonantcircuit with the output frequency of said auxiliary oscillator and tosuppress both of said frequencies, means for comparing the lowersideband output frequency of said modulator with the output frequency ofsaid variable oscillator, and means for connecting the output of saidvariable oscillator to an external circuit.

3. In a high frequency measuring apparatus, a precision oscillatoroperating at a fixed frequency substantially lower than that of a signalto be measured, a variable oscillator tunable over a range encompassingthe frequency of the signal to be measured, harmonic generator meansconnected to the output of said precision oscillator for generatingoscillations harmonically related to its fixed frequency, a tunableresonant circuit coupled to the output of said generator means,frequency-calibrated dial means for simultaneously adjusting thefrequency of said variable oscillator and the tuning of said circuitsuch that the latter resonates at a frequency a predetermined amounthigher than the adjusted frequency of said variable oscillator, anauxiliary oscillator tunable over a frequency band at least as great asthe frequency of said precision oscillator and which band includes afrequency equal to said predetermined amount, frequency-calibrated dialmeans for tuning said auxiliary 0scillator, modulator means connected tomodulate the output frequency of said resonant circuit with the outputfrequency of said auxiliary oscillator and to suppress both of saidfrequencies, and means for comparing the output of said modulator withthe output frequency of said variable oscillator.

4. Apparatus in accordance with claim 3, in which said respective dialmeans are located with their calibration indications adjacent oneanother so as to provide a single indication comprising the reading ofthe first dial means plus the reading of the second dial means.

5. In a high frequency measuring apparatus, a first variable oscillatortunable over a predetermined range, a precision fixed-frequencyoscillator, a frequency multiplier controlled by said precisionoscillator, a tunable output circuit connected to resonate saidfrequency multiplier in the region of a desired multiple of thefrequency of said precision oscillator, unicontrol means for varying thefrequency of said Variable oscillator and for simultaneously adjustingthe tuning of said multiplier output circuit to a frequency differing bya fixed amount from the frequency of the variable oscillator, a secondvariable oscillator covering a band Width equal to the frequency of saidprecision oscillator, which band includes a frequency equal to saidfixed amount, lattice modulator means connected to the output of saidfrequency multiplier and arranged to modulate the said output with theoutput of said second variable oscillator to produce sidebandfrequencies, and means for comparing one of said sideband frequencieswith the output of said first variable oscillator.

6. The invention in accordance with claim 3, in which thefrequency-calibrated dial means for tuning said auxiliary oscillatorcomprises a pair of indicating dials geared to one another for rotationin the same direction. 1

7. The invention in accordance with claim 6, in which the dial means arealigned for reading their respective indications at a common point as asingle multi-digit indication of frequency.

8. The invention in accordance with claim 1, in which said modulatormeans comprises a balanced, lattice-type modulator circuit.

9. The invention in accordance with claim 1, in which said modulatormeans comprises a balanced lattice-type modulator with a crystal dioderectifier element in each branch thereof.

10. In a high frequency measuring apparatus, a precision oscillatoroperating at a fixed frequency substantially different from that of asignal to be measured, a variable oscillator tunable over a rangeencompassing the frequency of the signal to be measured, harmonicgenerator means connected to the output of said precision oscillator forgenerating oscillations harmonically related to its fixed frequency, atunable resonant circuit coupled to the output of said generator means,means for simultaneously adjusting the frequency of said variableoscillator and the tuning of said circuit such that the latter resonatesat a frequency a predetermined amount different from the adjustedfrequency of said variable oscillator, an auxiliary oscillator tunableover a frequency band at least as great as the frequency of saidprecision oscillator, modulator means connected to modulate the outputfrequency of said resonant circuit With the output frequency of saidauxiliary oscillator and to suppress both of said frequencies, and meansfor comparing one of the sideband output frequencies of said modulatorwith the output frequency of said variable oscillater.

.11. In a high frequency measuring apparatus, a first variableoscillator tunable over a predeter mined range, a precisionfixed-frequency oscillator, a frequency multiplier controlled by saidprecision oscillator, a tunable output circuit connected to resonatesaid frequency multiplier in the region of a desired multiple of thefrequency of said precision oscillator, unicontrol means for varying thefrequency of said variable oscillator and for simultaneously adjustingthe tuning of said multiplier output circuit to a frequency differing bya fixed amount from the frequency of the variable oscillator, a secondvariable oscillator covering a band width equal to the frequency of saidprecision oscillator, balanced modulator means connected to the outputof said frequency multiplier and arranged to modulate the said outputwith the output of one of said variable oscillators to produce sidebandfrequencies, and means for comparing one of said sideband frequencieswith the output of the other of said variable oscillators.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,934,879 Potter Nov. 14, 1933 2,186,182 Stocker Jan. 9, 19402,324,077 Goodale et a1 July 13, 1943 2,398,694 Case Apr. 16, 19462,401,481 Harnett June 4, 1946 2,451,320 Clammer Oct. 12, 1948 2,491,494Grimm Dec. 20, 1949 FOREIGN PATENTS Number Country Date 596,889 GreatBritain Jan. 13, 1948

